Microsoft developed several sets of Visio stencils for OpsMgr that we use throughout System Center Operations Manager 2007 Unleashed. These have been posted on System Center Forum, at http://www.it-jedi.net/downloads/scomstencils.zip (new link attached on 2/9/09 as the previous link was no longer functional).
March 2011 update: The it-jeti.net link no longer works. The stencils have been relocated to http://www.systemcentercentral.com/Downloads/DownloadsDetails/tabid/144/IndexID/7556/Default.aspx. Note that only registered users may download. Registration is free!
There is also an additional set of stencils that we developed for the book, available with the companion CD distributed with the book.